Wireless and Mobile Networks Background: # wireless (mobile) phone - - PowerPoint PPT Presentation

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Wireless and Mobile Networks

Background:

 # wireless (mobile) phone subscribers now exceeds #

wired phone subscribers (5-to-1)!

 # wireless Internet-connected devices equals #

wireline Internet-connected devices

• laptops, Internet-enabled phones promise anytime untethered

Internet access

 two important (but different) challenges

• wireless: communication over wireless link
• mobility: handling the mobile user who changes point of

attachment to network

2

Outline

1 Introduction

Wireless

2 Wireless links, characteristics

• CDMA

3 IEEE 802.11 wireless LANs (“Wi-Fi”) 4 Cellular Internet Access

• architecture
• standards (e.g., GSM)

Mobility

5 Principles: addressing and routing to mobile users 6 Mobile IP 7 Handling mobility in cellular networks 8 Mobility and higher-layer protocols 9 Summary

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Elements of a wireless network

network infrastructure

4

wireless hosts

 laptop, smartphone  run applications  may be stationary (non-

mobile) or mobile

• wireless does not always

mean mobility

Elements of a wireless network

network infrastructure

5

base station

 typically connected to

wired network

 relay - responsible for

sending packets between wired network and wireless host(s) in its “area”

• e.g., cell towers,

802.11 access points

Elements of a wireless network

network infrastructure

6

wireless link

 typically used to connect

mobile(s) to base station

 also used as backbone

link

 multiple access protocol

coordinates link access

 various data rates,

transmission distance

Elements of a wireless network

network infrastructure

7

Characteristics of selected wireless links

Indoor

10-30m

Outdoor

50-200m

Mid-range

• utdoor

200m – 4 Km

Long-range

• utdoor

5Km – 20 Km

.056 .384 1 4 5-11 54

2G: IS-95, CDMA, GSM 2.5G: UMTS/WCDMA, CDMA2000 802.15 802.11b 802.11a,g 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 4G: LTWE WIMAX 802.11a,g point-to-point

200

802.11n

Data rate (Mbps)

8

infrastructure mode

 base station connects

mobiles into wired network

 handoff: mobile changes

base station providing connection into wired network

Elements of a wireless network

network infrastructure

9

ad hoc mode

 no base stations  nodes can only

transmit to other nodes within link coverage

 nodes organize

themselves into a network: route among themselves

Elements of a wireless network

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Wireless network taxonomy

single hop multiple hops

infrastructure (e.g., APs) no infrastructure

host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger

• Internet. May have to

relay to reach other a given wireless node MANET, VANET

IoT architecture

MN MN MN MN MN AN AN AN

IEEE 802.3, (ethernet) IEEE 802.11 (WiFi) …… IEEE 802.16e (WiMAX) 3G, 4G

MN = mobile node /network AN = ambient node / network

Moving clusters of nodes with sensing capability but often with limited resources e.g. single person moving around with IoT infrastructure: static ambient nodes attaching to clusters in lowest layer IP-based network with wide-area coverage, wireless

• r wired and

powerful servers sensors

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Taxonomy

Middle layer Bottom layer Multi hop (ambient infrastructure) Single hop (no hop) (access points) Multi hop Most general case:

• moving clusters

through ambient infrastructure

• ad-hoc networks
• Moving clusters

connecting to access points

• ad-hoc networks

Single hop (no hop) Moving nodes connecting to ambient infrastructure Moving nodes connecting to access points

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Examples

Middle layer Bottom layer Multi hop (ambient infrastructure) Single hop (no hop) (access points) Multi hop

• vehicle to vehicle
• vehicle to infra structure
• user wearing cluster of

sensors connected to phone

• vehicle to infra

structure

• user wearing

sensors connected to phone Single hop (no hop)

• phone
• sensor in low power

mesh network

• laptop / wifi

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Outline

1 Introduction

Wireless

2 Wireless links, characteristics

• CDMA

3 IEEE 802.11 wireless LANs (“Wi-Fi”) 4 Cellular Internet Access

• architecture
• standards (e.g., GSM)

Mobility

5 Principles: addressing and routing to mobile users 6 Mobile IP 7 Handling mobility in cellular networks 8 Mobility and higher-layer protocols 9 Summary

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Wireless Link Characteristics (1)

important differences from wired link ….

• decreased signal strength: radio signal attenuates as it

propagates through matter (path loss)

• interference from other sources: standardized wireless

network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well

• multipath propagation: radio signal reflects off objects

ground, arriving ad destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult”

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Wireless Link Characteristics (2)

 SNR: signal-to-noise ratio

• larger SNR – easier to

extract signal from noise (a “good thing”)

 SNR versus BER tradeoffs

• given physical layer: increase

power -> increase SNR- >decrease bit error rate (BER)

• given SNR: choose physical layer

that meets BER requirement, giving highest throughput

• SNR may change with

mobility: dynamically adapt physical layer (modulation technique, rate)

10 20 30 40

QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) SNR(dB) BER

10-1 10-2 10-3 10-5 10-6 10-7 10-4

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Wireless network characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access):

A B C

Hidden terminal problem

 B, A hear each other  B, C hear each other  A, C can not hear each other

means A, C unaware of their interference at B

A B C

A’s signal strength

space

C’s signal strength

Signal attenuation:

 B, A hear each other  B, C hear each other  A, C can not hear each other

interfering at B

Characteristics of Wireless Networks

 Multiplexing: in a mobile and wireless network, the

wireless medium is shared by many nodes.

 Hence, multiple use of a shared medium is a

major challenge in wireless networking.

 Most decisions for accessing the wireless medium

is made in the MAC layer.

Multiplexing

 The wireless channels can be multiplexed in four

dimensions:

• Time(t): A channel gets the whole frequency spectrum for a

certain amount of time.

• Space(s): Same frequency can be reused when the base

stations are separated in space.

• Frequency(f): The whole spectrum is separated into smaller

frequency bands.

• Code(c): Each channel uses a unique code for transmitting.

Time Division Multiplex (TDM)

 A channel gets the whole

frequency spectrum for a certain amount of time.

 Only one user for the

medium at a time.

 Usually the throughput is

high even with many users.

 However, no two users

should use the medium at the same time. Precise synchronization is needed.

t f

Space multiplexing : Cellular Networks

 Same frequency can be

reused when the base stations are separated in space.

 The reuse of frequencies

depend on signal propagation range.

 Example : fixed frequency

assignment for reuse with distance 2.

Frequency Division Multiplex (FDM)

 The whole spectrum is

separated into smaller frequency bands.

 A band is allocated to a

channel for the whole time.

 This is somewhat

inflexible if the traffic is non-uniform.

 An example is radio or

TV broadcast. The bandwidth is wasted if a station is off the air.

t f

Code Division Multiplex (CDM)

 Each channel uses a

unique code for transmitting.

 All channels use the same

frequency spectrum at the same time.

 However, signal

regeneration is very complex and requires complex HW/SW support.

f t c

Code Division Multiplexing

 CDMA has ben adopted for the 3G mobile phone

technology.

 CDMA is not very suitable for ad hoc networking

as we cannot expect specialized hardware/software support at the nodes.

 TDMA and its variations are most suitable for ad

hoc networking.

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Code Division Multiple Access (CDMA)

 unique “code” assigned to each user; i.e., code set

partitioning

• all users share same frequency, but each user has own

“chipping” sequence (i.e., code) to encode data

• allows multiple users to “coexist” and transmit

simultaneously with minimal interference (if codes are “orthogonal”)

 encoded signal = (original data) X (chipping

sequence)

 decoding: inner-product of encoded signal and

chipping sequence

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CDMA encode/decode

slot 1 slot 0

d1 = -1

1 1 1 1 1

• 1
• 1
• 1
• Zi,m= di.cm

d0 = 1

1 1 1 1 1

• 1
• 1
• 1
• 1 1 1

1 1

• 1
• 1
• 1
• 1 1 1

1 1

• 1
• 1
• 1
• slot 0

channel

• utput

slot 1 channel

• utput

channel output Zi,m sender

code data bits

slot 1 slot 0

d1 = -1 d0 = 1

1 1 1 1 1

• 1
• 1
• 1
• 1 1 1

1 1

• 1
• 1
• 1
• 1 1 1

1 1

• 1
• 1
• 1
• 1 1 1

1 1

• 1
• 1
• 1
• slot 0

channel

• utput

slot 1 channel

• utput

receiver

code received input Di = S Zi,m.cm

m=1 M

M

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CDMA: two-sender interference

using same code as sender 1, receiver recovers sender 1’s original data from summed channel data! Sender 1 Sender 2 channel sums together transmissions by sender 1 and 2

An example of TDMA

 CSMA/CD: Carrier Sense Multiple Access with

Collision Detection

 When a node wants to broadcast, it checks

whether any other node is broadcasting (senses the carrier).

 A node broadcasts when no other node is

broadcasting. Otherwise, it tries later at a random interval.

CSMA Problems in Wireless Medium

 Collision detection is easy in wired networks but difficult

in wireless medium.

 Collision avoidance to reduce wasted transmissions

S

Transmission Range of S

R

Message Loss due to Collision

 Using CSMA in wireless medium results in

message loss and requires retransmission of lost messages.

 A node spends much more energy while receiving

• r transmitting messages. Hence, retransmission

wastes a lot of energy.

 With only one antenna/radio, nodes can only listen

• r send.

 Full duplex radios are extremely expensive.  CSMA gives rise to hidden terminal and exposed

terminal problems.

CSMA Problems in Wireless Medium

Hidden Terminal Problem

R S2 S1

• Other senders’ information are hidden

from the current sender, so that transmissions at the same receiver cause collisions.

CSMA/CA

 Use of additional signaling packets

• Sender asks receiver whether it is able to receive a

transmission - Request to Send (RTS)

• Receiver agrees, sends out a Clear to Send (CTS)
• Sender sends, receiver Acknowledgements (ACKs)

S1 R S2

RTS CTS ACK DATA

time R S2 S1

1 2 3 4

Detect Collision Find Transmission Complete

Exposed Terminal Problem

R1 S1 S2 R2

• The sender mistakenly think

the medium is in use, so that it unnecessarily defers the transmission.

RTS CTS DATA

R1

time

S1 S2 R2

Find medium in use

1 2 3

Wait until medium is clear

CSMA – continued

 Synchronization

• There is a global clock. Every node knows the current time.
• There is a global schedule. Every node knows the schedule.

Name Time Tom 1:00 pm – 2:00 pm Peter 2:00 pm – 3:00 pm John 3:00 pm – 4:00 pm

Exposed Terminal

CSMA – continued

 When a node hears an RTS from a

neighboring node, but not the corresponding CTS, that node can deduce that it is an exposed terminal and is permitted to transmit to

• ther

neighboring nodes.

R1 R2 S1 S2

RTS CTS DATA

S2 R1

time

S1 R2

RTS CTS

t1 t2 t3 t4 t5 t6

DATA

1 2 3 4 5 6

Exposed Terminal

CSMA – continued

 Collision handling

• If a packet is lost (collision), the node back off

for a random time interval before retrying

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Outline

1 Introduction

Wireless

2 Wireless links, characteristics

• CDMA

3 IEEE 802.11 wireless LANs (“Wi-Fi”) 4 Cellular Internet Access

• architecture
• standards (e.g., GSM)

Mobility

5 Principles: addressing and routing to mobile users 6 Mobile IP 7 Handling mobility in cellular networks 8 Mobility and higher-layer protocols 9 Summary

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IEEE 802.11 Wireless LAN

802.11b

 2.4-5 GHz unlicensed spectrum  up to 11 Mbps  direct sequence spread spectrum

(DSSS) in physical layer

• all hosts use same chipping

code 802.11a

• 5-6 GHz range
• up to 54 Mbps

802.11g

• 2.4-5 GHz range
• up to 54 Mbps

802.11n: multiple antennae

• 2.4-5 GHz range
• up to 200 Mbps

 all use CSMA/CA for multiple access  all have base-station and ad-hoc network versions

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802.11 LAN architecture

 wireless host

communicates with base station

• base station = access point

(AP)

 Basic Service Set (BSS) (aka

“cell”) in infrastructure mode contains:

• wireless hosts
• access point (AP): base

station

• ad hoc mode: hosts only

BSS 1 BSS 2 Internet hub, switch

• r router

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802.11: Channels, association

 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels

at different frequencies

• AP admin chooses frequency for AP
• interference possible: channel can be same as that chosen

by neighboring AP!

 host: must associate with an AP

• scans channels, listening for beacon frames containing AP’s

name (Service Set Identification - SSID) and MAC address

• selects AP to associate with
• may perform authentication
• will typically run DHCP to get IP address in AP’s subnet

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802.11: passive/active scanning

AP 2 AP 1 H1 BBS 2 BBS 1

1 2 3 1

passive scanning:

(1) beacon frames sent from APs (2) association Request frame sent: H1 to selected AP (3) association Response frame sent from selected AP to H1

AP 2 AP 1 H1 BBS 2 BBS 1

1 2 2 3 4

active scanning:

(1) Probe Request frame broadcast from H1 (2) Probe Response frames sent from APs (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent from selected AP to H1

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IEEE 802.11: multiple access

 avoid collisions: 2+ nodes transmitting at same time  802.11: CSMA - sense before transmitting

• don’t collide with ongoing transmission by other node

 802.11: no collision detection!

• difficult to receive (sense collisions) when transmitting due to weak

received signals (fading)

• can’t sense all collisions in any case: hidden terminal, fading
• goal: avoid collisions: CSMA/C(ollision)A(voidance)

space

A B C A B C

A’s signal strength C’s signal strength

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frame control duration address 1 address 2 address 4 address 3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seq control

802.11 frame: addressing

Address 2: MAC address

• f wireless host or AP

transmitting this frame Address 1: MAC address

• f wireless host or AP

to receive this frame Address 3: MAC address

• f router interface to

which AP is attached Address 4: used only in ad hoc mode

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Internet router H1 R1 AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame R1 MAC addr H1 MAC addr

• dest. address

source address

802.3 frame

802.11 frame: addressing

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frame control duration address 1 address 2 address 4 address 3 payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seq control Type From AP Subtype To AP More frag WEP More data Power mgt Retry Rsvd Protocol version

2 2 4 1 1 1 1 1 1 1 1 duration of reserved transmission time (RTS/CTS) frame seq # (for RDT) frame type (RTS, CTS, ACK, data)

802.11 frame: more

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802.11: mobility within same subnet

 H1 remains in same

IP subnet: IP address can remain same

 switch: which AP is

associated with H1?

• self-learning: switch

will see frame from H1 and “remember” which switch port can be used to reach H1

H1 BBS 2 BBS 1

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802.11: advanced capabilities

Rate adaptation

 base station, mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies

QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps)

10 20 30 40

SNR(dB) BER

10-1 10-2 10-3 10-5 10-6 10-7 10-4

• perating point
• 1. SNR decreases, BER

increase as node moves away from base station

• 2. When BER becomes too

high, switch to lower transmission rate but with lower BER

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power management

 node-to-AP: “I am going to sleep until next

beacon frame”

• AP knows not to transmit frames to this node
• node wakes up before next beacon frame

 beacon frame: contains list of mobiles with AP-

to-mobile frames waiting to be sent

• node will stay awake if AP-to-mobile frames to be

sent; otherwise sleep again until next beacon frame

802.11: advanced capabilities

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M radius of coverage S S S P P P P M S

Master device Slave device Parked device (inactive)

P

802.15: wireless personal area network (WPAN)

 less than 10 m diameter  replacement for cables (mouse,

keyboard, headphones)

 ad hoc: no infrastructure  master/slaves:

• slaves request permission to send

(to master)

• master grants requests

 802.15: evolved from Bluetooth

specification

• 2.4-2.5 GHz radio band
• up to 721 kbps

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Outline

1 Introduction Wireless 2 Wireless links, characteristics

• CDMA

3 IEEE 802.11 wireless LANs (“Wi-Fi”) 4 Cellular Internet access

• architecture
• standards (e.g., GSM)

Mobility 5 Principles: addressing and routing to mobile users 6 Mobile IP 7 Handling mobility in cellular networks 8 Mobility and higher-layer protocols 9 Summary

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Mobile Switching Center

Public telephone network

Mobile Switching Center

Components of cellular network architecture

 connects cells to wired tel. net.  manages call setup (more later!)  handles mobility (more later!)

MSC

 covers geographical

region

 base station (BS)

analogous to 802.11 AP

 mobile users attach to

network through BS

 air-interface: physical

and link layer protocol between mobile and BS

cell

wired network

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Cellular networks: the first hop

Two techniques for sharing mobile-to-BS radio spectrum

 combined FDMA/TDMA:

divide spectrum in frequency channels, divide each channel into time slots

 CDMA: code division multiple

access

frequency bands time slots

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BSC

BTS

Base transceiver station (BTS) Base station controller (BSC) Mobile Switching Center (MSC) Mobile subscribers Base station system (BSS)

Legend

2G (voice) network architecture

MSC

Public telephone network

Gateway MSC G

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3G (voice+data) network architecture

radio network controller MSC SGSN

Public telephone network

Gateway MSC G

Serving GPRS Support Node (SGSN) Gateway GPRS Support Node (GGSN)

Public Internet

GGSN G

Key insight: new cellular data network operates in parallel (except at edge) with existing cellular voice network

 voice network unchanged in core  data network operates in parallel

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radio network controller MSC SGSN

Public telephone network

Gateway MSC G

Public Internet

GGSN G

radio access network

Universal Terrestrial Radio Access Network (UTRAN)

core network

General Packet Radio Service (GPRS) Core Network

public Internet radio interface

(WCDMA, HSPA)

3G (voice+data) network architecture

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Outline

1 Introduction Wireless 2 Wireless links, characteristics

• CDMA

3 IEEE 802.11 wireless LANs (“Wi-Fi”) 4 Cellular Internet Access

• architecture
• standards (e.g., GSM)

Mobility 5 Principles: addressing and routing to mobile users 6 Mobile IP 7 Handling mobility in cellular networks 8 Mobility and higher-layer protocols 9 Summary

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What is mobility?

 spectrum of mobility, from the network perspective: no mobility high mobility

mobile wireless user, using same access point mobile user, passing through multiple access point while maintaining ongoing connections (like cell

phone)

mobile user, connecting/ disconnecting from network using DHCP.

wide area network

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Mobility: vocabulary

home network: permanent “home” of mobile

(e.g., 128.119.40/24)

permanent address: address in home network, can always be used to reach mobile

e.g., 128.119.40.186

home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote

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Mobility: more vocabulary

wide area network

care-of-address: address in visited network.

(e.g., 79,129.13.2)

visited network: network in which mobile currently resides (e.g., 79.129.13/24) permanent address: remains constant (e.g., 128.119.40.186) foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile

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How do you contact a mobile friend:

 search all phone books?  call her parents?  expect her to let you

know where he/she is? I wonder where Alice moved to?

Consider friend frequently changing addresses, how do you find her?

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Mobility: approaches

 let routing handle it: routers advertise permanent address of

mobile-nodes-in-residence via usual routing table exchange.

• routing tables indicate where each mobile located
• no changes to end-systems

 let end-systems handle it:

• indirect routing: communication from correspondent to

mobile goes through home agent, then forwarded to remote

• direct routing: correspondent gets foreign address of

mobile, sends directly to mobile

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 let routing handle it: routers advertise permanent address of

mobile-nodes-in-residence via usual routing table exchange.

• routing tables indicate where each mobile located
• no changes to end-systems

 let end-systems handle it:

• indirect routing: communication from correspondent to

mobile goes through home agent, then forwarded to remote

• direct routing: correspondent gets foreign address of

mobile, sends directly to mobile

not scalable to millions of mobiles

Mobility: approaches

wide area network

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Mobility: registration

end result:

 foreign agent knows about mobile  home agent knows location of mobile

home network visited network

1

mobile contacts foreign agent on entering visited network

2

foreign agent contacts home agent home: “this mobile is resident in my network”

65

Mobility via indirect routing

wide area network

home network visited network

3 2 4 1 correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent foreign agent receives packets, forwards to mobile mobile replies directly to correspondent

66

Indirect Routing: comments

 mobile uses two addresses:

• permanent address: used by correspondent (hence

mobile location is transparent to correspondent)

• care-of-address: used by home agent to forward

datagrams to mobile

 foreign agent functions may be done by mobile itself  triangle routing: correspondent-home-network-

mobile

• inefficient when

correspondent, mobile are in same network

67

Indirect routing: moving between networks

 suppose mobile user moves to another network

• registers with new foreign agent
• new foreign agent registers with home agent
• home agent update care-of-address for mobile
• packets continue to be forwarded to mobile (but

with new care-of-address)

 mobility, changing foreign networks transparent: on

going connections can be maintained!

1 2 3 4

68

Mobility via direct routing

home network visited network

correspondent requests, receives foreign address of mobile correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile mobile replies directly to correspondent

69

Mobility via direct routing: comments

 overcome triangle routing problem  non-transparent to correspondent: correspondent

must get care-of-address from home agent

• what if mobile changes visited network?

1 2 3 4

70

wide area network

1

foreign net visited at session start anchor foreign agent

2 4

new foreign agent

3

correspondent agent correspondent new foreign network

Accommodating mobility with direct routing

 anchor foreign agent: FA in first visited network  data always routed first to anchor FA  when mobile moves: new FA arranges to have

data forwarded from old FA (chaining)

5

71

Outline

1 Introduction Wireless 2 Wireless links, characteristics

• CDMA

3 IEEE 802.11 wireless LANs (“Wi-Fi”) 4 Cellular Internet Access

• architecture
• standards (e.g., GSM)

Mobility 5 Principles: addressing and routing to mobile users 6 Mobile IP 7 Handling mobility in cellular networks 8 Mobility and higher-layer protocols 9 Summary

72

Mobile IP

 RFC 3344 – IP Mobility Support for IPv4  has many features we’ve seen:

• home agents, foreign agents, foreign-agent registration,

care-of-addresses, encapsulation (packet-within-a- packet)

 three components to standard:

• indirect routing of datagrams
• agent discovery
• registration with home agent

73

Mobile IP: indirect routing

Permanent address: 128.119.40.186 Care-of address: 79.129.13.2 dest: 128.119.40.186

packet sent by correspondent

dest: 79.129.13.2 dest: 128.119.40.186

packet sent by home agent to foreign agent: a packet within a packet

dest: 128.119.40.186

foreign-agent-to-mobile packet

74

Mobile IP: agent discovery

 agent advertisement: foreign/home agents advertise

service by broadcasting ICMP messages (typefield = 9)

RBHFMGV bits reserved type = 16 type = 9 code = 0 = 9 checksum = 9 router address standard ICMP fields mobility agent advertisement extension length sequence # registration lifetime

0 or more care-of- addresses

8 16 24

R bit: registration required H,F bits: home and/or foreign agent

75

Mobile IP: registration example

visited network: 79.129.13/24 home agent HA: 128.119.40.7 foreign agent COA: 79.129.13.2 mobile agent MA: 128.119.40.186

registration req.

COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification:714 ….

registration reply

HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format ….

registration reply

HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 ….

time ICMP agent adv.

COA: 79.129.13.2 ….

registration req.

COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format ….

76

Components of cellular network architecture

correspondent MSC MSC MSC MSC MSC wired public telephone network

different cellular networks,

• perated by different providers

recall:

77

Handling mobility in cellular networks

 home network: network of cellular provider you

subscribe to (e.g., Sprint PCS, Verizon)

• home location register (HLR): database in home network

containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)

 visited network: network in which mobile currently

resides

• visitor location register (VLR): database with entry for

each user currently in network

• could be home network

78

Public switched telephone network mobile user home Mobile Switching Center

HLR

home network visited network correspondent Mobile Switching Center

VLR

GSM: indirect routing to mobile

1 call routed to home network 2

home MSC consults HLR, gets roaming number of mobile in visited network

3 home MSC sets up 2nd leg of call to MSC in visited network 4 MSC in visited network completes call through base station to mobile

79

Mobile Switching Center

VLR

• ld BSS

new BSS

• ld

routing new routing

GSM: handoff with common MSC

 handoff goal: route call via

new base station (without interruption)

 reasons for handoff:

• stronger signal to/from new BSS

(continuing connectivity, less battery drain)

• load balance: free up channel in

current BSS

• GSM doesnt mandate why to

perform handoff (policy), only how (mechanism)  handoff initiated by old BSS

80

Mobile Switching Center

VLR

• ld BSS

1 3 2 4 5 6 7 8

new BSS

• 1. old BSS informs MSC of impending

handoff, provides list of 1+ new BSSs

• 2. MSC sets up path (allocates resources)

to new BSS

• 3. new BSS allocates radio channel for

use by mobile

• 4. new BSS signals MSC, old BSS: ready
• 5. old BSS tells mobile: perform handoff to

new BSS

• 6. mobile, new BSS signal to activate new

channel

• 7. mobile signals via new BSS to MSC:

handoff complete. MSC reroutes call 8 MSC-old-BSS resources released

GSM: handoff with common MSC

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home network Home MSC PSTN correspondent MSC anchor MSC MSC MSC

(a) before handoff

GSM: handoff between MSCs

 anchor MSC: first MSC

visited during call

• call remains routed

through anchor MSC

 new MSCs add on to end of

MSC chain as mobile moves to new MSC

 optional path minimization

step to shorten multi-MSC chain

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home network Home MSC PSTN correspondent MSC anchor MSC MSC MSC

(b) after handoff  anchor MSC: first MSC

visited during call

• call remains routed

through anchor MSC

 new MSCs add on to end of

MSC chain as mobile moves to new MSC

 optional path minimization

step to shorten multi-MSC chain

GSM: handoff between MSCs

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Mobility: GSM versus Mobile IP

GSM element Comment on GSM element Mobile IP element

Home system Network to which mobile user’s permanent phone number belongs Home network Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR) Home MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information Home agent Visited System Network other than home system where mobile user is currently residing Visited network Visited Mobile services Switching Center. Visitor Location Record (VLR) Visited MSC: responsible for setting up calls to/from mobile nodes in cells associated with

• MSC. VLR: temporary database entry in

visited system, containing subscription information for each visiting mobile user Foreign agent Mobile Station Roaming Number (MSRN), or “roaming number” Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent. Care-of- address

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Wireless, mobility: impact on higher layer protocols

 logically, impact should be minimal …

• best effort service model remains unchanged
• TCP and UDP can (and do) run over wireless, mobile

 … but performance-wise:

• packet loss/delay due to bit-errors (discarded packets,

delays for link-layer retransmissions), and handoff

• TCP interprets loss as congestion, will decrease congestion

window un-necessarily

• delay impairments for real-time traffic
• limited bandwidth of wireless links

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Summary

Wireless

 wireless links:

• capacity, distance
• channel impairments
• CDMA

 IEEE 802.11 (“Wi-Fi”)

• CSMA/CA reflects wireless

channel characteristics

 cellular access

• architecture
• standards (e.g., GSM, 3G,

4G LTE)

Mobility

 principles: addressing,

routing to mobile users

• home, visited networks
• direct, indirect routing
• care-of-addresses

 case studies

• mobile IP
• mobility in GSM

 impact on higher-layer

protocols